In Britain, the Duchess of Cambridge has been admitted to hospital in the early stages of labour. Kate, who is thought to be more than a week past her due date, is believed to have been taken through a side entrance of the Lindo wing at London's St Mary's Hospital early this morning.
The Duke of Cambridge travelled with her by car from Kensington Palace.
Sky News' Royal Correspondent Paul Harrison said her condition had been described as "very well".
He said "There's been a lot of speculation this morning since about 5.30am. One or two people had seen a convoy of vehicles that looked as though they had come from Kensington Palace. It seems it was a very orderly arrival. There were two vehicles. A Ranger Rover-type vehicle, and behind it there was an Audi vehicle".
"Prince William had said from the beginning that he wanted to be alongside his wife when she went into labour. The big wait begins. We don't know how long she will be in labour."
Kate is being tended by a top medical team led by the Queen's former gynaecologist Marcus Setchell, who delivered the Countess of Wessex's two children.
Former Midwife Clare Byam-Cook said "The Duke and Duchess will be as excited as any other new parents. I'm sure she'll be looked after to perfection. They'll be monitoring her very carefully to check that things are progressing well and that the baby is doing well."
The world's press have been camped outside St. Mary's in Paddington for days in anticipation of the birth. The Duke and Duchess have been at Kensington Palace all weekend and she went into labour naturally.
#Royalbaby is also top trending on Twitter:
Writing on Twitter, Kate's uncle, Gary Goldsmith, says today is "very very exciting". #RoyalBaby
— Peter Hunt (@BBCPeterHunt) July 22, 2013
Kensington Palace: The Duchess of Cambridge’s labour is progressing as normal #RoyalBaby
— Max Foster (@MaxFosterCNN) July 22, 2013
The press outside St Mary's in London where Kate Middleton is in labour via @rtenews #RoyalBaby pic.twitter.com/Q8hS4NeVCr
— SPIN 1038 (@spin1038) July 22, 2013
The #royalbaby will be duke and duchess's first child and will be third in line to the throne http://t.co/riZ8eP0iff
— BBC Breaking News (@BBCBreaking) July 22, 2013
The news was confirmed in a brief statement from the palace at 7.30am after rumours she had been spotted began to spread.
The Duke and his younger brother Prince Harry were born in the Lindo wing and the Prince and Princes of Wales famously posed on the building's steps in 1982 holding baby William. The Duke is known to want a daughter, while the Duchess is hoping for a son.
Betting on the name of their first-born, which will be third-in-line to the throne, has produced one favourite with a number of bookies - Alexandra. Whatever name they go with, it is likely to set a trend for the next generation of infants.
Recent changes to the rules of succession mean that if it is a girl, she will not be leapfrogged to the throne by a younger brother.
The arrival of the royal baby will be announced with a notice behind the iron railings of Buckingham Palace.
The brief bulletin, on headed Palace notepaper, confirms the sex of the baby but usually gives little else away other than that the baby has been "safely delivered" and perhaps the weight.
When the child is born, Prince William will be on paternity leave from his job as an RAF search and rescue helicopter pilot. It is not known how long Kate intends to put her royal duties on hold.
